Evaporative cooling of polymer composition



Jan. 20, 1970 w. G. JOHNSON 3,4914177 EVAPORTIVE-COOLING 0F POLYMER COMPOSITION Filed March s1, 1967 IN VENTOR MAL/HN G JOHA/soAf) United States Patent O 3,491,177 EVAPORATIVE COOLING OF POLYMER COMPOSITION William G. Johnson, Augusta, Ga., assignor to E. I. du

Pont de Nemours and Company, Wilmington, Del., a corporation of Delaware Filed Mar. 31, 1967, Ser. No. 627,543

Int. Cl. D01d 5/08; B29c 25/00, 17/14 U.S. Cl. 264-143 6 Claims ABSTRACT OF THE DISCLOSURE This invention relates generally to the production of fiber-forming polymers and is directed more particularly to an improved process useful in the cooling of freshly extruded molten structures.

A process for the production of polymer ribbon, generally suitable for conversion to flake, has been disclosed by Graves in U.S. Patent No. 2,289,774. One type of apparatus for facilitating a rapid quenching has been disclosed by La Forge in U.S. Patent No. 3,071,810. Another improvement has been disclosed by Jones in U.S. Patent No. 3,227,519, which is particularly useful for polymers showing a greater than normal tendency to remain soft and tacky. After drying, blending, and other preparatory steps, such Iflake is usually fed into the hopper of a meltspinning apparatus. One of the major difficulties involved in these known procedures has been the length of time required for casting the extruded ribbon. In this respect, it has also been recognized that a faster casting cycle would lead to the production of polymer having a more uniform thermal history and generally improved quality. A faster casting cycle has generally been obtained by the addition of more spray cooling devices, roughened ribbon surfaces, and quenching tanks, all of which are satisfactory in decreasing casting time but result in flake which has a higher and/ or more variable moisture content than desirable for melt spinning. This requires additional drying of the flake to remove the excessive moisture.

The most important .object of this invention is to provide a process in which the casting cycle time is reduced without increasing the moisture level of the flake produced.

The object of the invention is accomplished by a process which comprises extruding a molten synthetic polymeric composition in the form of a ribbon onto a casting wheel for conveying the ribbon past one or more nozzles directed to spray a cooling fluid onto the ribbon so that the coolant evaporates freely on the surface of the ribbon. If desired, the ribbon may be further cooled by passage through a quenching tank filled with cooling fluid. The cooling fluid is then stripped from the ribbon. After the ribbon passes the air jet strippers to remove the cooling fluid, a reapplication of cooling fluid is made by spray nozzles before the ribbon reaches the cutter. The reapplication of cooling 3,491,177 Patented Jan. 20, 1970 ice fluid may be made to one or both sides of the ribbon. It is most surprising that the addition of a coolant fluid, such as water, after the air strippers does not in fact increase the moisture level .of the resultant flake and does improve the moisture uniformity of the flake and the uniformity of casting time from batch to batch. With no intent of being held to this explanation, it is believed that coolant fluid is removed by evaporation aided by a combination of (l) the heat produced in the cutting machine through friction and (2) the air moving through the cutting machine.

The process for carrying out the invention is shown diagrammatically in the accompanying .drawing in which FIGURE 1 is a -flow sheet indicating the steps included between the production of the molten polymer and its formation into flake; FIGURE 2 is a view in elevation showing the formation of the ribbon from the molten polymer, the quenching of the ribbon, the stripping, spraying, and cutting of the ribbon.

The improvement over the processes of the prior art involves the reapplication of cooling fluid after the ribbon has been stripped of excessive moisture to provide a more rapid and uniform casting of the ribbon without increasing the moisture level of the resultant flake.

In operation, referring to FIGURE 2, molten polymer in autoclave 1 is forced through extruder 2 by means such as inert gas pressure to form a ribbon 3, which is deposited on casting wheel 4. As the casting Wheel turns, the ribbon is carried past the spray nozzles 5, the ribbon is removed by stripper roll 6, and advanced to quench tank 7. After 1 or more trips through the fluid in tank 7, the ribbon is withdrawn over guide roll 8, and dried by means of air jets 9. Additional cooling is provided by spray nozzles 10. Guide roll 11 transports the ribbon 3 into the cutter 12 for cutting the ribbon into flake, which is collected in drum 13. The temperature is measured during the casting cycle by a thermometer or suitable device inserted in the flake stream discharged from the cutter 12. The speed of the casting cycle is adjusted to maintain the desired temperature of the flake. The speed is decreased if the temperature of the flake becomes too high and increased if the temperature becomes too low.

The invention is further illustrated by the following specific example:

EXAMPLE I Polyhexamethylene adipamide, prepared as described in U.S. 2,163,636, is extruded from an autoclave downwardly through an extrusion die at a rate of about lbs./min. onto a casting wheel. The molten polymer is extruded at 275 C. through a 7/32 by 17f1/2 in. slot. The flow rate is controlled by a valve located immediately above the extrusion die both of which are electrically heated to maintain the polymer temperature. Molten polymer coming from the slot flows through a few inches of air space onto a water-cooled drum Where it solidifies, and is stripped from the drum. After the ribbon is stripped from the casting drum, it is further quenched in a pool of water and dried by means of air jets. Water is sprayed on one face of the ribbon by means of conventional water-spray nozzles to provide a uniform film of water on that face. After the spraying step, the ribbon is fed directly to a cutter where the ribbon is cut into flake. The temperature of the flake is measured continuously during the cycle. Additional batches are cast in a similar manner except that the final water-spray is eliminated in one case,

and the spray and air stripping of water in the other. The

results are as follows: f

TABLE I i Casting Casting Time (min.) Moisture, Percent Number Temp.

Type of cooling Batehes C.) Avg. r Avg. a

Pool Quench Only 4 90-100 17. 9 1. 9 0. 39 0. 15 Pool Quench-l-Air Strip 9 105 21. 7 0. 61 0. 26 0. 037 Pool Quench-i-Air Strip-l-Rewct-I- Evap. Cool 8 95405 1G. 9 0. 71 0. 30 0. 026

From these data it is readily seen that the use of the water sprays after air stripping produces the shortest average casting time having the best uniformity of moisture content as measured by Sigma, the standard deviatron.

The amount of water added by means of the sprays after the air strippers was found to be immaterial so long as a uniform film coated at least one surface of the ribbon.

It will be appreciated by those skilled in the art that variations from the details given can be made without departing from the scope of the present invention.

What is claimed:

1. In a process for producing ilake from molten polyhexamethylene adipamide including the steps of (1) extruding the said polyhexamethylene adipamide in the form of a ribbon,

(2) conveying the said ribbon past a first spray means,

(3) spraying the said ribbon with cooling water from said rst spray means,

(4) stripping the said water from said ribbon, and cutting said ribbon into ilake, the improvement comprising reapplying cooling water to the said ribbon by a second spray means after said stripping has ccurred then cutting the ribbon into flake.

2. Claim 1 wherein said second spray means is a spray nozzle.

3. Claim 2 wherein said ribbon is extruded onto a casting wheel for conveying the said ribbon past said rst spray means, and said stripping is accomplished by air jets.

4. In a process for producing ake from molten polyhexamethylene adipamide including the steps of (2) conveying the said ribbon past a first spray means,

(3) spraying the said ribbon with water from said iirst spray means,

(4) 'passing the said ribbon through a quench tank containing water,

(5) stripping the water from said ribbon, and cutting the ribbon to flake, the improvement comprising reapplying water to the said ribbon by a second spray means after said stripping has occurred then cutting the ribbon into flake.

5. Claim 4 wherein said second spray means is a spray nozzle.

y6. Claim 4 wherein said ribbon is extruded onto a casting wheel for conveying the said ribbon past said rst spray means, and said stripping is accomplished by air jets.

References Cited UNITED STATES PATENTS 2,859,476 11/1958 Eckardt 264-178 3,046,083 7/1962 Bates et al 264-143 3,154,605 10/1964 Meyer et al. 264-216 3,207,828 9/ 1965 Petersen et al. 264-233 `3,283,378 11/1966 Cramton 264-210 3,324,217 6/1967 Armstrong et al. 264-210 JULIUS FROME, Primary Examiner HERBERT MINTZ, Assistant Examiner U.S. Cl. X.R. 

